The present invention relates generally to detecting surges that occur in rotating compressors and, more particularly, for a method and means to quickly detect surge and generate control signals which are used to prevent serious compressor damage caused by excessive surging by providing warnings and control actions before the individual and cumulative stresses of repetative surge cycles damage the compressor.
Turbo compressors are used to deliver compressed gas into many and varying type processes. These processes impose a resistance to flow. The resistance may be relatively constant, or it may vary considerably during normal or abnormal process operation. An increase in process resistance causes an increase in the compressor discharge pressure. If the resistance becomes excessive, a point is reached where the compressor is not capable of producing the necessary discharge pressure and a momentary flow reversal occurs. This flow reversal is called "surge". It can cause serious compressor damage due to induced vibratory stresses and very high temperatures. During normal operation the discharge gas is hot due to the compression process. When surge occurs the hot gas flows back through the compressor causing the inlet temperature to increase. Work is actually done on the gas during the "surge" flow reversal, so the resultant inlet temperature can increase to a value even higher than the discharge temperature had been just before the flow reversal. This is particularly true of an axial flow type turbo compressor, which also is more susceptible to surge damage. Therefore, the invention is of particular value for axial flow compressors. This phenomenon may be repeated at frequent internals and a high mechanical stress is placed upon the blades and bearings which can cause serious compressor damage. The potentially damaging effect cannot be precisely measured, but is a function of the number, magnitude and duration of the surge cycles.
The main protection against surging is the use of an antisurge control mechanism which, at some limiting point prior to surge, opens bypass valves to vent the compressor discharge to the atmosphere to keep the rate of fluid flow in the compressor at some admissable value. A surge detection system, as in this invention, is used as a backup in case the main protection system fails to prevent surge. Various methods of surge detection were used in the prior art.
In some instances a single temperature sensor such as a thermocouple is located at the compressor intake to detect the sudden temperature rise which accompanies the surge. In such case, the system must be set to operate higher than the highest normal operating temperature in order to detect the abnormal temperature rise. For example, a 150.degree. F. set point would be typical for a maximum normal operating temperature of 100.degree. F. Further, with the single temperature sensor the time to detect surge increases as the operating temperature decreases. For example, an increase of 150.degree. F. would be needed to activate the surge detector when the compressor is operating at 0.degree. F. Such a system would have failed to count medium and mild surges because the change in temperature was less than 150.degree. F.
Other prior art systems use a pressure differential or rate of change in pressure or flow to detect surge. See U.S. Pat. No. 4,046,490. These systems using pressure change as the detector must be set to operate over a rate of change indicative of surge while ignoring normal rates of change. The proper setting cannot be accurately calculated, therefore actual compressor surge tests are necessary to assure correct setting.
As stated earlier, some prior art systems had only one thermocouple in the compressor. Others have one in the compressor and another located to sense gas temperature in the inlet pipe upstream from the compressor. Automatic controls were required to disable the surge detection system when the compressor was shut down. This was necessary to prevent false surge alarms when a compressor thermocouple is heated due to temperature soaking from the hot discharge into the compressor. Still other surge detectors use the vibration of the compressor to detect the occurrence of a surge as disclosed in U.S. Pat. No. 4,399,548. Here the surge must progress to a certain degree of intensity before the vibration is serious enough to be detected.
The present invention provides faster and more reliable surge detection and responds to all surges which cause greater than a 50.degree. F. increase in temperature regardless of the operating temperature at which the surge occurs. Also, the system response is extremely fast and reacts to start corrective action less than 1/4 of a second after the onset of a surge. Further, set points for this invention at which signals are developed representing magnitude of surge are not rate-of-change dependent and thus surge tests are not necessary. Also, no automatic controls are required to disable the surge detection system when the compressor is shut down because the heat soaking is not fast enough to produce a change in temperature alarm required by the present case. Further, it provides a warning signal or compressor shut-down based upon the number, intensity and duration of the surges, not just the number of them.
The present invention utilizes two thermocouples located in the compressor inlet such that both thermocouples are subject to the common gas inlet temperature. One of the thermocouples has a rapid response, Tf, to temperature change and the other thermocouple has a slow response, Ts, to temperature change in comparison with the first thermocouple rapid response. These thermocouples are connected electrically in opposed relationship thereby producing a signal output, Tf-Ts, for a given change in temperature. Thus with any rapid temperature change accompanying a surge, a differential signal is produced in proportion to the temperature change whereby the differential signal may be used to detect the number, magnitude and duration of the surges which are occurring. Of course the signal Tf+Ts could be used to indicate a rapid change in termperature. If so, the signal levels would have to change in the control circuitry. Thus the present invention provides the following unique features:
1. Provides a time delay in energizing alarm/counting circuits to prevent false alarm/counts when the power supply is turned on.
2. Provides "fail safe" action in the event of a disconnected or broken wire in the control circuit.
3. Provides an alarm if the thermocouple detection circuit breaks.
4. Counts mild surges.
5. Counts medium surges.
6. Counts major surges.
7. Provides an analog signal for every surge cycle. This signal is indicated and also can be recorded to provide a permanent record of all surges and their relative intensity. The signal peak valve increases with surge intensity.
8. Provides a surge countdown from a predetermined initial setting, with the number of counts per surge varied according to the time period (relative intensity) of each surge cycle.
9. Provides an alarm warning that the compressor internals should be inspected for evidence of damage when the accumulated surge induced stresses reach a predetermined value.
10. Provides a signal to open a discharge vent valve to eliminate the surge condition.
11. Provides a signal to stop the compressor when surge occurs.
12. Provides full time surge detection, i.e. the system is operable throughout startup, and the critical shutdown phase of operation, whereas it was necessary to disable systems utilizing prior art to avoid false surge alarm/counts during startup and shutdown.